1. Field of the Invention
The present invention relates to a process for producing metal oxide particles having a very small and uniform size. More particularly, the present invention relates to a process for producing very small and uniform metal oxide particles with a high productivity and efficiency by oxidizing metal vapor in a turbulent diffusing flame.
2. Description of the Prior Art
It is well known that fine particles of various metal oxides, for example, magnesium oxide and calcium oxide, exhibit excellent heat resistance and electrical insulating property and, therefore, are highly useful as ceramic materials, catalysts, pigments, or fillers in a wide range of industries. Especially, it was recently discovered that very fine metal oxide particles having a very small size of 0.1 .mu.m or less exhibit various unique properties different from those of coarse metal oxide particles. For example, very fine metal oxide particles exhibit excellent chemical reactivity due to their large total surface area and the high surface energy of the particles. Also, very fine metal oxide particles exhibit different magnetic and optical properties from those of usual metal oxide particles in the form of a bulk due to the very small volume of the individual particles.
The above-mentioned specific properties open up new fields of application for very fine metal oxide particles, for example, starting materials for catalysts, sintered materials, porous materials, sensor materials, magnetic materials, and pigments.
It is known that the fine metal oxide particles can be produced by a liquid phase reaction method or a gas phase reaction method.
In the liquid phase reaction method, a metal salt is deposited from its aqueous solution and the deposited metal salt is collected and thermally decomposed to form corresponding metal oxide particles. This method is, however, not usually utilized to produce metal oxide particles having a small size of 0.1 .mu.m or less, because the resultant fine metal oxide particles easily agglomerate to form secondary agglomerates having a large size during the process.
In the gas phase reaction method, it is believed that very fine metal oxide particles can be produced by carrying out the metal oxide-forming reaction under appropriate conditions, because, in this method, the resultant fine metal oxide particles do not easily agglomerate, the formation of secondary agglomerates is very small, and the reaction conditions can be easily decided.
The gas phase reaction method can be classified into a first method, wherein metal vapor is brought into contact with an oxygen-containing gas at a temperature at which the metal vapor can be oxidized into fine metal oxide particles, and a second method, wherein metal oxide particles are produced in a combustion flame generated by the combustion of a corresponding metal substance which is capable of being oxidized.
In the first gas phase reaction method, for example, metallic magnesium is heated within an inert gas atmosphere to generate magnesium vapor, the magnesium vapor is flowed into an oxidizing region, and a flow of a molecular oxygen-containing gas is introduced into the oxidizing region countercurrently to the flow of the magnesium vapor so as to allow the magnesium vapor to contact and react with the molecular oxygen-containing gas. This method for producing fine magnesium oxide particles having a high purity is disclosed in Czechoslovakian Pat. No. 139,208.
Also, Takanori Watari, Kazumi Nakayoshi and Akio Kato, Journal of Japanese Chemical Society, No. 6, pages 1075 to 1076 (1984), disclose a process for producing fine magnesium oxide particles, in which process metallic magnesium is heated, and the resultant magnesium vapor is introduced together with argon gas into a reactor and is mixed with an oxygen (O.sub.2) nitrogen (N.sub.2) mixture gas.
In the above specifically mentioned processes, in order to produce very fine metal oxide particles having a very small size, it is usually necessary to dilute the metal vapor with a large amount of an inert gas and then to bring the diluted metal vapor-containing gas into contact with a molecular oxygen-containing gas. This process is disadvantageous judging from the very high production cost of the resultant fine metal oxide particles and the very complicated and expensive production apparatus.
The afore-mentioned second method, wherein metal oxide particles are produced by oxidizing a metal substance capable of being converted into a corresponding metal oxide in a combustion flame, is classified into a premixing-combustion method and a diffusion combustion method.
In the premixing-combustion method, a combustible metal substance-containing gas and a molecular oxygen-containing gas are mixed and the mixed gas is ejected through a burner into an oxidizing region. In this method, for example, a metal-halogen compound which is capable of being converted into a corresponding metal oxide is mixed with a combustible material gas, for example, hydrogen gas or methane gas, which are converted into hydrogen oxide and/or carbon dioxide by combustion, and the resultant mixed gas is burned by using a watering can type burner to generate a number of thin flames. This process is disclosed in Japanese Examined Patent Publication (Kokoku) No. 36-3359. This method is, however, disadvantageous because a number of fiber-shaped metal oxide agglomerates are formed around the nozzles of the burner and the size of the resultant metal oxide particles is not even.
In order to produce metal oxide particles having uniform size, it was attempted to use a highly reactive starting material such as metal vapor in place of the metal halide compound having a relatively low reactivity. In the case where a metal halide compound is used, the reaction of the metal halide compound with a molecular oxygen-containing gas is carried out at a moderate reaction rate due to the relatively low reactivity of the metal halide compound. In the case where metal vapor is used, however, the reaction of the metal vapor with the molecular oxygen-containing gas is immediately initiated upon mixing and progresses at such a very high reaction rate that it is difficult to control the reaction rate to an appropriate level. Even if the reaction rate at the initial stage of the reaction at which the metal vapor is mixed with the molecular oxygen-containing gas in a mixer and at which the mixed gas is fed into a burner could be controlled, there is a high possibility of dangerous backfires from the burner to the mixer. Due to the above-mentioned possibility, in the premixing-combustion method, the starting metal material should be a metal compound having a relatively low reactivity and a high reactive starting material such as metal vapor cannot be utilized.
The diffusion-combustion method generally relates to a process in which a combustible gas and a molecular oxygen-containing gas are ejected into an oxidizing region through separate ejecting nozzles and the ejected combustible gas and molecular oxygen-containing gas contact and mix with each other to generate a diffusing flame in the oxidizing region to burn the combustible gas.
Usually, the diffusion-combustion method is applied to ordinary combustion of LPG or heavy oil. It has never been applied to the production of metal oxides.
The diffusion-combustion method includes a laminar flow diffusing flame method and a turbulent flow diffusing flame method.
In the laminar flow diffusing flame method, a combustible gas contacts a molecular oxygen-containing gas under a laminar flow diffusing condition and is burnt in a resultant laminar flow diffusing flame. In this laminar flow diffusing flame, the combustible gas and the molecular oxygen-containing gas, which are in a laminar flow condition, have an interface formed therebetween and diffuse through the interface into each other. The result is a laminar flow type flame having an interface formed between a combustible gas-diffused molecular oxygen flame and a diffused combustible gas-molecular oxygen-containing gas flame.
According to the present inventor's research, the laminar flow diffusing flame has a relatively large length. This feature causes the residing time of the resultant oxide particles in the flame to be long. Therefore, the resultant oxide particles grow in the flame, and the size of the grown particles is undesirably large and uneven. For example, the residing (growing) time of a metal oxide particle produced at a location close to the rear end of the flame is remarkably longer than that produced at a location close to the front end of the flame. This fact results in a large distribution range of the sizes of the resultant metal oxide particles.
Accordingly, in order to produce very small and uniform metal oxide particles by the laminar flow diffusing flame method, it is necessary to limit the partial pressure of the metal vapor in the metal-oxidizing region to a very low level, for example, 0.09 atmosphere, and the reaction temperature to a low level, for example, 800.degree. C. to 1,600.degree. C. This necessity sometimes causes the efficiency of the metal oxide production to be unsatisfactorily low. This type of process is disclosed by Japanese Unexamined Patent Publication (Kokai) No. 59-213619.